The rendering of wood less susceptible to fungal and bacterial deterioration has been a major concern for many years. The rotting of wood by fungal and/or bacterial degradation is a particularly distressing problem, especially in relation to the use of wood for construction purposes and more especially with respect to the construction of homes.
Three classes of wood additives have proved to be the main rotproofing agents for wood for many years, i.e., creosote compounds, pentachlorophenol and arsenic compounds. While being relatively effective rotproofing additives, their use commercially has faded because they are all potential carcinogens. Not only are those treating the wood to render it rotproof exposed to these carcinogens, but in those cases where the treated wood is used in buildings, those humans or animals living in those buildings may also be in danger. Furthermore, since these additives are held in the wood by physical forces only, they oftimes migrate to the surface of the wood. Therefore, the inner portions of the wood are usually not durably treated--especially in the case of the use of creosote and/or pentachlorophenol.
In order to produce more durable rotproofing, wood has also been treated with phenol-formaldehyde precondensates or prepolymers which polymerize or cure to resinous products inside the wood. These products, however, also have disadvantages. They have a tendency to polymerize in solution before and during treatment, especially under conditions which catalyze the polymerization of the phenol-formaldehyde within the wood. When the molecule of precondensate or prepolymer is sufficiently large, it does not readily penetrate through the pores of the wood and especially will not penetrate into the core or heartwood. To alleviate this situation and to further improve the rotproofing characteristics, pentachlorophenol is oftimes added to the aqueous solution of phenol-formaldehyde resin former. Not only does this addition further the problem of the carcinogenicity because of the pentachlorophenol, but it adds to the cost of treating the wood because of the cost of the pentachlorophenol and the solvent for the pentachlorophenol which is necessary to make it miscible with the aqueous solution of phenol-formaldehyde precondensate. Also, since the pentachlorophenol does not react with the phenol-formaldehyde, it can leach out of the wood. U.S. Pat. No. 4,399,195 is directed to a composition of this type wherein the solvent is methanol, and over which this invention is an improvement.
Much of the rotproofing of wood today entails the use of pressure treatment of the wood to force the molecules, if they are small enough, into the core or heartwood. See, for example, U.S. Pat. No. 3,968,276, incorporated herein by reference. The pressure treatments are carried out in containers which are capable of being evacuated as well as pressurized. Oftimes the vessels containing the wood are first evacuated to remove air and moisture from the interior of the wood. With aqueous solutions of phenol-formaldehyde precondensate, green wood can be treated because some of the water can be removed, especially with cycles of evacuation to permit sufficient phenol-formaldehyde resin solids to be forced into the wood by the pressure treatment following the vacuum treatment.
The vacuum step may be carried out under various pressures for various time periods. Normally, vacuum of at least 10 inches of mercury and up to 30 inches of mercury or even more is used with time periods usually over 10 to 15 minutes and for longer periods of time especially if green wood is being treated. After evacuation the treating solution is added and then pressure applied. The pressure and time of treatment also can be varied. Normally, at least 100 psi is used and can be increased to as high as 300 psi or even higher. Times of pressure can vary from one to two minutes up to several hours.
The use of formaldehyde, urea-formaldehyde resins, melamine-formaldehyde resins, and resins of formaldehyde and various unsubstituted phenols and chlorinated phenols as rotproofing agents for regenerated cellulose, i.e., viscose rayon, is taught by Bell et al; J. Soc. Dyers & Colourists 71, November 1955; pages 660-667. Similar treatments of cotton are shown by Chance et al; Textile Research Journal, July 1959; pages 558-564.
Cotton and viscose rayon, however, while both being cellulosic in nature, are materially different than wood with regard to the size of specimen being treated and the difficulty most additives have in penetrating into the deepest portions of the wood. The Bell et al article teaches the use of many prepolymers which are converted into resins once present in the substrate. Although reaction of the prepolymer, i.e., monomer, with the cellulose is postulated, reaction of the prepolymer to form a resin still requires the availability of the reactive methylol groups capable of forming such a resin. Polymerization of phenol-formaldehyde prepolymers occurs either between available methylol groups on two different prepolymer molecules or between a methylol group and a hydrogen group of two different prepolymer molecules. In both instances, methylol groups available for reaction with a reactive hydroxy group of cellulose are taken up in the resin formation. Thus, reduction in the permanency of the resin positioning within the wood may occur. Bell et al further indicate that chlorinated phenols, however, fail to form resins when added to rayon as prepolymers.
Chance et al show the use of phenol, hydroxybenzyl alcohol, chlorinated phenols, brominated phenols, and fluorinated phenols as additives to cotton wherein again a resin is subsequently formed, and as such, the products resulting therefrom suffer from the same deficiencies as discussed above regarding viscose rayon. Chance, in fact, indicates that resin penetration into the cellulosic fiber is very poor, i.e., only a surface treatment is achieved.